(a) Technical Field
The present invention relates to a fuel cell separator with a gasket and a method for manufacturing the same. More particularly, it relates to a fuel cell separator with a gasket which can further improve airtightness, electrical stability, and durability, and a method for manufacturing the same.
(b) Background Art
The configuration of a fuel cell stack may be described as follows with respect to a unit cell. A membrane-electrode assembly (MEA) is positioned in the center of each unit cell of the fuel cell stack. The MEA comprises a polymer electrolyte membrane, through which hydrogen ions (protons) are transported, and an electrode/catalyst layer such as an air electrode (cathode) and a fuel electrode (anode), in which an electrochemical reaction between hydrogen and oxygen takes place, disposed on each of both sides of the polymer electrolyte membrane.
Moreover, a gas diffusion layer (GDL) and a gasket are sequentially stacked on both sides of the MEA, where the cathode and the anode are located. A separator including flow fields for supplying fuel and discharging water produced by the reaction is located on the outside of the GDL. After a plurality of unit cells are stacked together, an end plate for supporting the above-described components is connected to the outermost ends of the fuel cell stack, thereby completing the manufacturing the fuel cell stack.
Therefore, at the anode of the fuel cell stack, hydrogen is dissociated into hydrogen ions (protons, H+) and electrons (e−) by an oxidation reaction of hydrogen. The hydrogen ions and electrons are transmitted to the cathode through the electrolyte membrane and the separator, respectively. At the cathode, water is produced by the electrochemical reaction in which the hydrogen ions and electrons transmitted from the anode and the oxygen in air participate and, at the same time, electrical energy is produced by the flow of electrons.
The separator (especially, a metal separator) of the fuel cell stack includes flow fields formed by stamping a thin metal plate having a thickness of about 0.1 mm to supply a reducing gas and an oxidizing gas to the fuel cell stack, supply coolant for cooling the fuel cell stack, and collect and transmit generated electricity. Therefore, the separator should have airtightness and liquid-tightness such that the reducing gas, the oxidizing gas, and the coolant are not mixed together.
Therefore, the gasket is applied to one side of the separator to maintain the airtightness of the coolant and reactant gases (hydrogen and air) and, at the same time, support the gasket disposed on the other side of the separator.
The gasket is integrally formed on both sides of a metal separator by injection molding in terms of productivity during manufacturing of the fuel cell stack. An example of this is described with reference to FIG. 1 and FIG. 2.
In particular, FIG. 1 is a cross-sectional view showing a conventional method of integrally bonding a gasket 7 to a separator by injection molding. First, an adhesive is applied to an area, where the gasket 7 is to be formed, of the entire surface of a metal separator 1 (hereinafter referred to as a separator).
Then, the separator 1 is loaded on the top of a lower mold 3 in such a manner that the separator 1 is positioned between an upper gasket groove 2a and a lower gasket groove 3a, which are configured to fit the shape of the gasket 7 in an upper mold 2 and the lower mold 3.
Subsequently, the upper mold 2 is moved to press and fix the edge of the separator 1, and then a gasket material is injected into the top and bottom of the separator 1 to integrally mold and bond a gas side gasket 5 and a cooling side gasket 6 to both sides of the separator 1.
FIG. 2 is a plan view and a cross-sectional view showing the structure of the conventional metal separator 1 integrated with the gasket 7 by injection molding. As shown in the figure, the gasket 7 is integrally formed on the edge of the metal separator 1 and on the periphery of manifolds. The gasket 7 comprises the gas side gasket 5 integrally formed on the top of the metal separator 1 and maintaining the airtightness of reactant gas (hydrogen or air) and the cooling side gasket 6 integrally formed on the bottom of the metal separator 1 and maintaining the airtightness of coolant.
As such, the gasket 7 formed on the separator serves as a basis for defining each unit cell of the fuel cell stack and seals hydrogen, coolant, and air flow fields, respectively, formed on the surface of the separator 1.
While the above-described method of forming the gasket 7 on both sides of the separator 1 by injection molding can manufacture the fuel cell stack in a simple manner, the following problems are encountered during injection molding of the gasket 7, which will be described with reference to FIGS. 3 and 4.
FIG. 3 is a cross-sectional view showing the occurrence of deformation and contamination of the separator 1 during injection molding of the gasket 7.
The structures and thicknesses of the gas side gasket 5 and the cooling side gasket 6 formed on both sides of the separator 1 are different from each other, and thus when the gasket material is injected on both sides of the separator 1 formed of a thin plate having a thickness of about 0.1 mm, the midsection of the separator 1 may be bent downward due to a pressure difference between the gasket materials being flowed through both sides of the separator 1.
For example, when the gasket material is simultaneously injected into the upper gasket groove 2a formed in the upper mold 2 and the lower gasket groove 3a formed in the lower mold 3, the amount of the gasket material filled in the upper gasket groove 2a having a large thickness is greater than that of the gasket material filled in the lower gasket groove 3a having a small thickness, and thus a pressure difference between the gasket materials filled in the upper gasket groove 2a and the lower gasket groove 3a occurs. As a result, the midsection of the separator 1 is bent downward due to the pressure difference.
Moreover, a non-filled portion may occur due to a difference in flow rate between the gasket materials being flowed through both sides of the separator 1. Otherwise, the gasket material leaks from the upper and lower molds 2 and 3, which results in the formation of burrs.
For example, when the gasket material is injected into the gasket grooves 2a and 3a at the same pressure, the flow rate of the gasket material being flowed through the upper gasket grooves 2a is lower than that of the gasket material being flowed through the lower gasket groove 3a, which results in the occurrence of the non-filled portion.
Moreover, when the pressure of the gasket material being flowed through the upper gasket groove 2a is higher than the pressure applied to the edge of the separator 1, into which no gasket material is injected, the gasket material leaks from a gap between the upper and lower molds 2 and 3 and is then solidified, which results in the formation of the burrs.
In addition, when the gasket material is introduced through the gap between the upper and lower molds 2 and 3, it contaminates the surface of the separator 1, which increases the contact resistance of the separator 1, thereby deteriorating the performance of the fuel cell stack.
When the burrs are present in the flow fields of the separator having a concave-convex structure, the removal of the burrs cannot be automated using a tool or die cutter, and thus the removal of the burrs should be performed manually.
FIG. 4 is a cross-sectional view showing the occurrence of short circuit and corrosion in a recess of a conventional separator integrated with a gasket.
It can be seen from FIG. 4 that the gasket 7 is not formed on the edge of the separator 1, and the metal surface of this edge of the separator 1 on which the gasket 7 is not formed is always exposed to the outside.
The reason that the gasket 7 is not formed on the edge of the separator 1 is that when the gasket 7 is integrally formed on the separator 1, the edge of the separator 1 is in contact with the upper and lower molds 2 and 3 and held thereby.
Of course, even though the gasket 7 is not formed on the edge of the separator 1, it has no significant effect on the airtightness performance.
However, the edge of the separator 1 is a dead zone which is not related to the performance of the fuel cell. When the area of the edge of the separator 1 is increased to reduce the deformation of the separator 1, which is caused by the pressure difference between the gasket materials being flowed through both sides of the separator 1, the power density of the fuel cell stack may be reduced.
In particular, a recess formed in an empty space between the edges of the separators 1 is most likely to be exposed to high temperature and high humidity conditions, which often occur during operation of the fuel cell stack, and in this case, condensed water is formed in the recess formed in the empty space between the edges of the separators 1.
As a result, the condensed water flows through the unit cells to form an electrical path, which results in the occurrence of short circuit between the unit cells. Moreover, the surface of the separator 1 is corroded by the condensed water, which reduces the durability of the separator 1.
Reference numeral 8 in FIG. 4 denotes a membrane-electrode assembly (MEA).
The above information disclosed in this Background section is only for enhancement of understanding of the background of the invention and therefore it may contain information that does not form the prior art that is already known in this country to a person of ordinary skill in the art.